Generally, an OTFT, which is a device for driving a next-generation display apparatus, is under active study. The OTFT includes, as a semiconductor layer, an organic film in lieu of a silicon film, and is classified into a monomer OTFT using oligothiophene, pentacene and so on, and into a polymer OTFT using polythiophene and so on, depending on the type of material of the organic film.
The OTFT is manufactured mainly using a solution process including dissolving an organic semiconductor in a solvent thus forming a thin film on a substrate. In the case where a multilayered thin film including a dielectric layer, an organic semiconductor layer and a protective layer is manufactured through the solution process, problems in which a solvent used in subsequent processes damages the under lying layers which have been already formed may occur. In order to more effectively perform the solution process, thorough research into manufacturing a multilayered thin film through a single coating process is being conducted. Also, to reduce the manufacturing cost, methods for using an expensive organic semiconductor in a small amount are in constant demand.
To this end, the combination of the electrical properties of the organic semiconductor, the mechanical properties of the insulating polymer, and the material properties of the inexpensive material is being studied. Recently, H. Sirringhaus Group, UK, has succeeded in the manufacture of a device able to maintain charge mobility even when poly-3-hexyl thiophene (P3HT) is used only in an amount of about 3 wt % upon blending of P3HT which is a kind of organic semiconductor with polystyrene (PS) or polyethylene (PE) which is an insulting polymer, and the results thereof are reported in Nature Materials, 5, 956 (2006) and disclosed in PCT patent WO 2008/001123 A1. However, in the case of this method, the device able to maintain charge mobility despite the use of a small amount of P3HT may be manufactured only under a condition in which the insulating polymer used is a crystalline polymer, for example, isotactic-PS or high-density PE. The reason is that P3HT is spread on the substrate while being crystallized, and the insulating polymer layer is then formed on the P3HT layer, thus obtaining a structure in which the P3HT layer and the insulating polymer layer are vertically phase-separated on the substrate. Accordingly, even when P3HT is used in a small amount, a charge transfer passage may be formed between source and drain electrodes. However, this method is difficult to commercialize because of a complicated manufacturing process including the crystallization of P3HT and then the solidification of the insulating polymer. Further, because a drop-casting process is employed, it makes it difficult to manufacture a device through application of a uniform film on a large area.
Using a blend of P3HT and polymethylmethacrylate (PMMA), the P3HT and PMMA are vertically phase-separated on the substrate so that PMMA is located on P3HT to thus use the PMMA layer as the protective layer of the P3HT layer, which was studied by A. Arias in Palo Alto Research Center (Adv. Mater. 19, 2900 (2006)). However, PMMA has a limitation of using it only as the protective layer, and also, P3HT should be added up to 40% and is thus unfavorable in terms of reducing use of the organic semiconductor.
In Adv. Mater, 2008, 20, 1141-1145 and PCT/KR2008/005427, in order to solve these problems, there have been proposed methods of coating the surface of a hydrophilic substrate with a blend solution of a polymer semiconductor and an insulating polymer having surface energy greater than that of the organic semiconductor to thus achieve vertical phase-separation. According thereto, the polymer semiconductor and the insulating polymer applied on the hydrophilic substrate through spin coating are configured such that the insulating polymer having relatively greater surface energy is formed on the hydrophilic substrate, and the polymer semiconductor having smaller surface energy is formed on the insulating polymer thin film. In this case, when the polymer semiconductor is used as the organic semiconductor layer and the insulating polymer is used as the dielectric layer, a high-performance OTFT may be manufactured through a single process.
However, the polymer semiconductor has low crystallinity and does not exceed field effect mobility of 0.1 cm2/Vs. In the spin casting process, the blend solution is subjected to strong centrifugal force, and the difference in concentration of the solution due to evaporation of the solvent causes Marangoni instability, resulting in lateral phase-separation. Hence, it is very difficult to control the phase separation process. Further, the two-layer structure of polymer/polymer blend is considerably unstable because it may be broken. Accordingly, the demand for methods of simply and reproducibly manufacturing the multilayered thin film composed of organic semiconductor layer/insulating layer through a solution process continues.
Therefore, the present invention is intended to provide methods of manufacturing a multilayered thin film using vertical phase-separation of a blend of small molecular organic semiconductor/insulating polymer and of increasing the phase separation of the thin film of the blend through an annealing process and the crystallinity of the organic semiconductor, and fabrication of the OTFT using the same.